ANTENNASANTENNAS
ANTENNA
A structure that is generally a metallic object, often a wire or group of wires, used
to convert high frequency signals into electromagnetic waves and vice versa.
A device whose function is to radiate electromagnetic energy and/or
intercept electromagnetic radiation
ANTENNA
Basic Consideration:
Maximum Power
Transfer
FIELDS OF AN
ANTENNA
INDUCTION FIELD
Considered to extend out from the antenna to a
distance of πD2/8λ
Near Field or Fresnel Region
RADIATION FIELD
Considered to extend out from a distance of
2D2/λ
Far Field or Fraunhoffer Region
TRANSITION ZONE
Zone between the two(2) regions
BASIC FORMULAS
T = 1/f
Where: T – time
F - frequency
λ = c/f
Where: λ – wavelength
F – frequency
C – velocity in free space
CHARACTERISTICS OF CHARACTERISTICS OF
ANTENNASANTENNAS
RADIATION PATTERN
A line drawn to join points in space which have equal field intensity due to the source.
Major Lobe – the direction of maximum radiation
Minor Lobe – the direction of minimum radiation
Null – the direction with radiation intensity equal
to zero.
a. Antenna height
b. Power losses
d. Thickness of the antenna wire
FACTORS WHICH DETERMINE THE
RADIATION PATTERN OF AN
ANTENNAc. Terminations
at its end
PRINCIPLE OF RECIPROCITY
The characteristics of antennas, such as impedance and radiation pattern are identical, regardless of use for
reception or transmission.
ISOTROPIC ANTENNA
An antenna that radiates uniformly in all directions in space
DIRECTIVE GAIN The ratio of the power density in a particular direction of one antenna to
the power density that would be radiated by an isotropic antenna.
HERTZIAN DIPOLE: 1.5 : 1 (1.76 dB)
HALF WAVE DIPOLE: 1.64 : 1 (2.15 dB)
NOTES:
The longer the antenna, the higher the directive gain
Non resonant antennas have higher directive gain than resonant
antennas
The directive gain of all practical antennas is greater than unity
DIRECTIVITY, D
Maximum directive gain
The gain in the direction of one of the major lobes in the antenna’s radiation pattern.
POWER GAIN
Overall gain considering losses and efficiency
Ap = %D
Where: % - antenna efficiency
D - directivity
ANTENNA RESISTANCE
The ratio of the applied voltage to
the flowing current
AC resistance
The ratio of the power radiated by the
antenna to the square of the current at the
feedpoint.
1. RADIATION RESISTANCE, Rr
Antenna and ground resistance
Discharge or corona effects
Losses in imperfect dielectric very near
the antenna
Eddy current loss
2. LOSS RESISTANCE, Rd
ANTENNA EFFICIENCY
The ratio of the power radiated by the antenna to the power delivered at the feedpoint
The ratio of radiation resistance to the total system resistance
% = Rr / Rr + Rd
An antenna has a radiation resistance of 72 ohms, a loss resistance of 8 ohms, and a power gain of 16. What efficiency and directivity does it have.a. 90% and 17.78b. 10% and 17.78c. 90% and 14.4d. 10% and 14.4
To produce a power density of 1 mw/m2 in a given direction, at a distance of 2 km, an antenna radiates a total of180 w. An isotropic antenna would have to radiate 2400 w to produce the same power density at that distance. What, in dB, is the directive gain of the practical antenna?a. 11.25 dBb. 13.21 dBc. 10 dBd. 6 dB
EFFECTIVE RADIATED POWER (ERP)
The product of the power fed to an antenna and its power gain.
ERP = Total Radiated Power x Power Gain
The power radiated by an antenna in its favored direction, taking the gain of the antenna
into account as referenced to an isotropic radiator
EFFECTIVE ISOTROPIC RADIATED POWER (EIRP)
FRONT TO BACK RATIO
Ratio of the power at the optimum direction of the antenna to that
of the power 180 degrees from the
optimum direction
The operating frequency range
of an antenna
BANDWIDTH
BEAMWIDTH
Angular separation between two half power points in a major lobeof an antenna radiation pattern
The degree of concentration of the antenna’s radiation
POLARIZATION
Space orientation of the waves that the antenna
radiates
The electric field vector is always parallel to the
antenna elements.
PHYSICAL LENGTH
Actual length of the antenna
λ/2 is the shortest length of a conductor which will
resonate at a given frequency
L =λ/2; λ= c/f
ELECTRICAL LENGTH
Dependent upon the velocity coefficient or velocity factor
where k is the dielectric constant
L = k λ
Ex. What is the wavelength of a 500 MHz signal?a. 60 cmb. 6 mc. 0.06 md. 60 m
Ex. What is the electrical length of an antenna operating at a frequency of 500 kHz?a. 500 mb. 570 mc. 600 md. 630 m
GROUNDING SYSTEMSGROUNDING SYSTEMS
EFFECTS OF GROUND ON ANTENNAS
Whereas an ungrounded antenna with its image forms an antenna
array, the bottom of the grounded antenna is joined to the top of the
image; the system acts as an antenna of double size.
EFFECTS OF GROUND ON ANTENNAS
Whereas an ungrounded antenna with its image forms an antenna array, the bottom of the grounded antenna is joined to the top of the image; the system acts as an antenna of
double size.
GROUND SCREEN
A network of buried wires directly under the antenna, consisting of a large number of radials extending
from the base of the tower, like spokes on a wheel, and placed 15 and 30 cm
below the ground.
COUNTERPOISE
A substitute for ground screen in areas of low conductivity, i.e. rock, mountains and antennas on top of
buildings
ANTENNA HEIGHTANTENNA HEIGHT
ANTENNA HEIGHT
The actual antenna height should at least be λ/4, but where this is not
possible, the effective height should correspond to λ/4.
TOP LOADING
A good method of increasing radiation resistance by having a horizontal portion at the top of the antenna
Effect: to increase the current at the base of the antenna and to make the current distribution more uniform
EFFECTIVE LENGTH
Antennas behave as though (electrically) they were longer than
their physical length
The result of physical antennas having finite thickness, instead of being
infinitely thin.
END EFFECT
ANTENNA COUPLING AND ANTENNA COUPLING AND
IMPEDANCE MATCHINGIMPEDANCE MATCHING
ANTENNA COUPLING
A network composed of reactances and transformers, which may be lumped or distributed, to provide
impedance matching
To tune out the reactive component of the antenna impedance
REASONS FOR
COUPLING To provide the transmitter with the correct value of load resistance
To prevent illegal radiation of spurious frequencies
ANTENNA COUPLERS
The antennas are coupled directly to their transmitters
Affords a wider reactance range, giving adequate harmonic suppression
Used for balanced lines
Direct Coupler
π Coupler
Symmetrical πCoupler
IMPEDANCE MATCHING
Accomplished by connecting the coax or twin lead to the stub and sliding the connections up or down the stub until
the proper SWR is indicated by a meter connected in the system.
Stub Matching
IMPEDANCE MATCHING
Accomplished by spreading the ends of the feedline and adjusting
the spacing until optimum performance is reached.
Delta MatchingDelta Matching
A sliding clamp is included in the assembly to permit fine
tuning for minimum SWR at the time of installation
Gamma Matching
IMPEDANCE MATCHING
A section of transmission line one quarter wavelength
long placed between the load and the line
Quarter Wave
Matching
BALUN Used to connect an unbalanced (coaxial line) to a balanced antenna
IMPEDANCE MATCHING
CURRENT FED (LOW Z FEED)
An antenna is said to be current fed if it is fed at the point of current maximum
Includes all feed point impedances below 600 ohms
Ex. Center fed half wave dipole or Marconi antenna
VOLTAGE FED (HIGH Z FEED)
An antenna is said to be voltage fed if it is fed at the point of voltage maximum
Includes all feed point impedances in excess of 600 ohms
Ex. Center fed full wave dipole
REVIEW QUESTIONSREVIEW QUESTIONS
1. Device that converts high frequency current into electromagnetic waves.
a. antennab. loudspeakerc. microphoned. lightning arrester
2. A polar diagram or graph representing field strengths or power densities at various angular positions relative to an antenna.
a. Venn Diagramb. Figure 8 patternc. Lissajous figured. Radiation Pattern
3. Refers to the orientation of the electric field radiated from an antenna.
a. radiationb. polarizationc. beamwidthd. bandwidth
4. Pertains to a wire structure placed below the antenna and erected above the ground which is a form of capacitive grounding system.
a. imageb. counterpoisec. antenna orientationd. polarization
5. What is the technique used to electrically increase the antenna length?
a. loadingb. using image antennac. using antenna arraysd. increasing antenna height
6. Antenna supported by insulators seems electrically longer than its physical length due to
a. imageb. reflectionc. end effectd. broadside effect
7. The ratio of the power radiated by the antenna to the total input power.
a. power gainb. directive gainc. antenna efficiencyd. radiation efficiency
8. The ratio of the front lobe power to the back lobe power
a. front to side ratiob. front to back ratioc. back to front ratiod. minor to major ratio
9. The standard reference antenna for directive gaina. infinitesimal dipoleb. isotropic antennac. elementary doubletd. half wave dipole
10. The gain of a hertzian dipole with respect to an isotropic antenna
a. 1.76 dBb. 2.15 dBc. 1.5 dBd. 1.64 dB
11. A half wave dipole antenna is capable of radiating 2000 watts and has a 2.15 dB gain over an isotropic antenna. How much power must be delivered to the isotropic antenna to match the field strength of the directional antenna?a. 1640 wattsb. 3280 wattsc. 4300 wattsd. 3520 watts
12. An ungrounded antenna near the grounda. acts as a single antenna of twice the heightb. is unlikely to need a ground screenc. acts as an antenna arrayd. must be horizontally polarized
13. Top loading is sometimes used with an antenna in order to increase itsa. effective heightb. bandwidthc. beamwidthd. input capacitance
14. Very low signal strength in an antenna
a. minor lobesb. nullsc. antenna patternsd. major lobes
15. A horizontal antenna is ______ polarized.a. verticallyb. horizontallyc. centrallyd. circularly
16. An antenna with unity gain.a. rhombicb. half wave dipolec. isotropicd. whip
17. What is the front to back ratio of an antenna which radiates 500 watts in a northernly direction and 50 watts in a southernly direction?a. 25000 dBb. 10 dBc. 100 dBd. 20 dB
18. Good grounding is important fora. Horizontal antennasb. Broadside arrayc. Vertical antennasd. Yagi Uda Antennas
19. If the radiated power increases 10.89 times, the antenna current increases by
a. 3.3 timesb. 6.6 timesc. 1.82 timesd. 10.89 times
20. Shortening effect of an antenna that makes it appear as if it were 5% longer
a. end effectb. flywheel effectc. skin effectd. capture effect
21. If an antenna is too short for the wavelength being used, the effective length can be increased by adding
a. capacitance in seriesb. inductance in seriesc. resistance in paralleld. resistance in series
22. Actual height of an antenna should be at leasta. 1λb. λ/2c. λ/4d. ¾ λ
23. The directivity pattern of an isotropic radiatora. figure 8b. a spherec. unidirectional cardioidd. parabola
24. A Hertz antenna is operating on a frequency of 2182 kHz and consists of a horizontal wire that is hanged between two towers. What is the frequency of its third harmonic?a. 727 kHzb. 6546 kHzc. 436 kHzd. 6.546 kHz
25. What is the gain of an antenna over a half wavelength dipole when it has 6 dB gain over an isotropic radiator?a. 6 dBb. 8.1 dBc. 3.9 dBd. 10 d
BASIC TYPES OF BASIC TYPES OF
ANTENNASANTENNAS
a standard reference antenna , radiating equally in all directions,
so that the radiation pattern is spherical.
ISOTROPIC ANTENNA ELEMENTARY DOUBLET
A theoretical antenna shorter than a wavelength used as a standard to
which all other antenna characteristics can be compared
9 = 60π le I sin θ / λr
Where
N – antenna length
r – antenna length
Le – antenna length
I – antenna current
θ – angle of axis and point of maximum radiation
Ex. An elementary doublet is 10 cm long. If the 10 MHz current flowing through it is 2 A, what is the field strength 20km away from the doublet in a direction of maximum radiation?a. 6.28 uV/mb. 62.83 uV/mc. 15.92 uV/md. 1.59 uV/m
An antenna made up of two wires bent at 90
degrees to each other so as to be in the same line and signal is fed at the center
DIPOLE
HALF WAVE DIPOLE
Length is λ/2 and radiation pattern is a toroid (bidirectional)
VOLTAGE AND CURRENT CHARACTERISTICS
NON-RESONANT ANTENNA
One in which there are no standing waves
Radiation pattern is directional
Standing waves are suppressed by the use of a correct termination to ensure that no power is reflected, so that only a forward traveling
wave will exist.
LONG WIRE ANTENNA
Lengths in the order of several wavelengths
When an antenna is 2 or more wavelengths long, it provides gain and a multilobe radiation
pattern.
When terminated at one end, it becomes unidirectional.
RHOMBIC ANTENNA
Consists of non-resonant antenna elements arranged differently, i.e. planar rhombus
Length of equal radiators = 2 to 8 ‘s
Angle of tilt: 40 to 75˚
Rt = 800 ohms
Rin = 650 to 700 ohms
Non-resonant antenna used for long distance sky wave transmission or reception of horizontally polarized
waves over distances from 200 to over 3000 miles at frequencies from 4 to 22
MHz.
RHOMBIC ANTENNA
RESONANT ANTENNA
Standing waves exist, caused by the presence of both a reflected traveling
wave and the forward wave.
Antenna whose length is a multiple of λ/4’s
HERTZ ANTENNA
An antenna system in which the ground is not an essential part
Half Wave Dipole
Half wave antenna used for frequencies above 2 MHz
MARCONI ANTENNA
Grounded Quarter Wavelength antenna
Vertical Monopole
Quarter Wavelength antenna used for frequencies below 2
MHz; omnidirectional
ANTENNA ARRAYSANTENNA ARRAYS
ANTENNA ARRAY
A radiating system consisting of individual
radiators or elements placed close together so
as to be within each other’s induction field
DRIVEN ELEMENT
Element of an array connected to the output
of the transmitter
Radiation not directly connected to the output of the
transmitter
PARASITIC ELEMENT
Receives energy through the induction field of a driven
element
REFLECTOR
A parasitic element longer than the driven element and close to it reduces signal strength in its own direction and
increases it in the opposite direction.
DIRECTOR
A parasitic element shorter than the driven
one from which it receives energy; tends to increase radiation in its
own direction
BROADSIDE ARRAY
Simplest array which consists of a number of dipoles of equal size, equally spaced along a straight line with all dipoles fed in the same phase from the same source.
Typical antenna length: 2 to 10 ‘s
Typical spacing: λ/2 or λ/4
Number of elements: dozens
ENDFIRE ARRAY
Physical arrangement is the same as that of the broadside array
The magnitude of the current in each element is still the same as in every other element, there is now a phase difference between these currents.
TURNSTILE ARRAY
Consists of two horizontal, half wave antennas mounted at right angles to each
other
YAGI UDA ANTENNA
An array consisting of a driven element and one or more parasitic elements arranged collinearly
and close together.
FOLDED DIPOLE
Single antenna which consists of 2 elements, one is fed directly and
the other coupled conductively at the
ends.
LOG PERIODIC ANTENNA
Main feature is frequency independence for both radiation resistance and pattern
Bandwidths of 10:1 are achievable with ease
Radiation patterns: uni and bidirectional
Main feature is frequency independence for both
radiation resistance and pattern
LOG PERIODIC ANTENNA
PYRAMIDAL ANTENNA
a type of log periodic antenna
It looks and works in much the same way as a standard log periodic
antenna, with one big difference: the two halves of the transmission line are
separated and positioned as a V, so each half of the transmission line is in effect a single wire transmission line.
UHF AND MICROWAVE UHF AND MICROWAVE
ANTENNASANTENNAS
PARABOLIC ANTENNA
Parabola – a plane curve defined as the locus of a point which moves so that its distance from another point (called the focus) plus its distance from a straight
line (directrix) is constant.
Works on the principle of a parabola
PARABOLIC ANTENNA
All waves coming from the source and reflected by the
parabola will travel in the same distance by the time they reach
the directrix, no matter from what point on the parabola they
are reflected.
CASSEGRAIN ANTENNA
an antenna in which the radiator is mounted at or near the surface of a
concave main reflector and is aimed at a convex secondary reflector slightly inside
the focus of the main reflector.
BEAMWIDTH
Ф = 70λ / D
Ф0 = 2 Ф
Where:
Ф0 = beamwidth between nulls, degrees
Ф = beamwidth between half power points, degrees
λ = wavelength, meters
D= mouth diameter, meters
GAIN OF A PARABOLIC ANTENNA
Ap = 6 ( D/λ)2
Where:
Ap = Power Gain
λ = wavelength, meters
D= mouth diameter, meters
Ex. Calculate the beamwidth between nulls of a 1m paraboloid reflector used at 6GHz.
a. 3.5˚b. 7˚c. 1.75˚d. 14˚
Ex. Calculate the gain of the paraboloid reflector in the previous problem.
a. 9600b. 2400c. 1600d. 2800
HORN ANTENNA
Ideal as primary feed antenna for parabolic reflectors and lenses
LENS ANTENNA
Used as a collimator of frequencies in excess of 3 GHz
HELICAL ANTENNA
Broadband VHF and UHF antenna which is used when it is desired to provide circular polarization characteristics
Consists of a loosely wound helix, backed up by a ground plane, which is simply a screen made of chicken wire
DISCONE ANTENNA
A combination of a disk and a cone in close proximity
Characterized by an enormous bandwidth for both input impedance and
radiation pattern
A constant angle, low gain antenna; omnidirectional
LOOP ANTENNA
Used for direction finding, because they do not radiate in a direction at right angles to the
plane of the loop.
For portable domestic receivers Circular or square shaped
PHASED ARRAY
Group of antennas, connected to one
transmitter or receiver, whose radiation beam can be adjusted electronically
without physically moving parts; used in radars.
WHIP ANTENNA
the most common example of a monopole antenna, an antenna with a single driven
element and a ground plane.
The whip antenna is a stiff but flexible wire mounted, usually vertically, with one end
adjacent to a ground plane.
SLOT ANTENNA
When the plate is driven as an antenna by a driving frequency, the
slot radiates electromagnetic waves in similar way to a dipole antenna.
consists of a metal surface, usually a flat plate, with a hole or slot cut out.
NOTCH ANTENNA
An open ended slot antenna
Since the currents are zero at the middle of the slot, we may cut the ground plane here to
make a notch antenna.
ANTENNA SUPPORT ANTENNA SUPPORT
STRUCTURESSTRUCTURES
POLE
Used to signify one piece of structure similar to the
common telephone pole
ANTENNA SUPPORTING STRUCTURES
Used to designate a structure made of
metal or wood which may be either in a form
of a one piece or sectionalized structure
MAST
Applied to a very large, high
structure, which in most cases, is constructed of
metal
TOWER
REVIEW QUESTIONSREVIEW QUESTIONS
1. A non-resonant antenna that is capable of operating satisfactorily over a relatively wide bandwidth, making it ideally suited for HF transmission.
a. end-fire arrayb. rhombicc. broadside arrayd. log periodic
2. Antenna that is independent of their radiation resistance and radiation pattern to frequency. It has bandwidth ratios of 10:1 or greater.
a. loop antennab. helicalc. Yagi Uda antennad. Log periodic antenna
3. A half wave antennaa. Hertzb. Marconic. Parabolicd. Vertical Monopole
4. Antenna which is used very frequently but almost entirely as a reception antenna and is usually found at the back of table radios.
a. loop antennab. folded antennac. rhombicd. log periodic
5. One of the special purpose antennas which has broadband VHF and UHF that is ideally suited for applications for which radiating circular rather than horizontal or vertical polarized electromagnetic waves are required.
a. loop antennab. phased arrayc. folded dipoled. helical
6. What are the two types of antenna elements?a. driven and reflectorb. director and reflectorc. parasitic and directord. driven and parasitic
7. Which one of the following terms does not apply to the Yagi Uda array?
a. good bandwidthb. parasitic elementsc. folded dipoled. high gain
8. Indicate the antenna that is not wideband.a. disconeb. folded dipolec. helicald. Marconi
9. One of the following is not an omnidirectional antennaa. halfwave dipoleb. log periodic antennac. disconed. Marconi
10. One of the following consists of nonresonant antennasa. rhombicb. folded dipolec. end fire arrayd. broadside array
11. Which of the following is best excited from a waveguide?
a. biconicalb. hornc. helicald. discone
12. An antenna that is circularly polarizeda. parabolic reflectorb. Yagi Udac. Helicald. Circular loop
13. What is the polarization of a discone antenna?a. verticalb. horizontalc. circulard. spiral
14. When speaking of antennas, ____ is a section which would be a complete antenna by itself.a. imageb. top loadingc. bayd. quarterwave
15. ______ is an antenna with a number of half wave antennas in it.
a. antenna arrayb. towerc. omnidirectionald. rhombic
16. Which antenna radiates an omnidirectional pattern in the horizontal plane with vertical polarization?a. Marconib. Disconec. Hornd. Helical
17. An antenna with very high gain and very narrow beamwidth.a. helicalb. disconec. hornd. parabolic dish
18. An open ended slot antennaa. helicalb. rhombicc. notchd. cassegrain
19. Which antenna is properly terminated?a. Marconib. Rhombicc. Dipoled. Yagi Uda
20. What is the radiation characteristic of a dipole antenna?a. omnidirectionalb. bidirectionalc. unidirectionald. hemispherical
21. An antenna which is one tenth wavelength long.a. hertz antennab. loop antennac. Marconi antennad. Elementary doublet
22. What is the minimum number of turns a helical antenna must have?a. 3b. 4c. 5d. 6
23. An antenna made up of a number of full wavelengthsa. elementary doubletb. log periodicc. long wired. whip
24. Which of the following improves antenna directivity?a. driven elementb. reflectorc. directord. parasitic element
25. The frequency of operation of a dipole antenna cut to a length of 3.4 ma. 88 MHzb. 44 MHzc. 22 MHzd. 11 MHz
26. Where are the voltage nodes in a half wave antenna?a. at the endsb. three quarters of the way from the feedpoint towards
the endc. one half of the way from the feedpoint towards the endd. at the feedpoint
27. A simple half wavelength antenna radiates the strongest signala. at 45 degrees to its axisb. parallel to its axisc. at right angles to its axisd. at 60 degrees to its axis
28. An antenna array which is highly directional at right angles to the plane of the arraya. Broadsideb. End firec. Turnstiled. Log periodic
29. What is the usual electrical length of a driven element in an HF beam antenna?a. λ/4b. λ/2c. 3/4 λd. 1 λ
30. The input terminal impedance at the center of a folded dipole antennaa. 72 Ωb. 300 Ωc. 50 Ωd. 73 Ω
31. The length of a Marconi antenna to operate with 985 kHz isa. 200 ftb. 500 ftc. 250 ftd. 999 ft
32. The purpose of stacking elements on an antennaa. sharper directional patternb. increased gainc. improved bandpassd. all of these
33. Which of the following antennas is used for testing and adjusting a transmitter for proper modulation, amplifier operation and frequency accuracy?a. elementary doubletb. realc. isotropicd. dummy
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